It is known that the headroom of an amplifier circuit is a fundamental limit to the gain that can be achieved for amplifying a signal. The “headroom” typically refers to the maximum output voltage swing of the amplifier above which the output voltage clamps. If the input signal has a dc component, typically referred to as a “dc offset,” then the gain of the amplifier would have to be limited in order not to exceed the allowable headroom once both the dc component and the alternating current (ac) component of the input signal are amplified by the amplifier circuit. The result is that there is an inability to optimize the ac gain due to the presence of the dc component.
Attempts at reducing input voltage offset in an amplifier circuit generally include blocking the dc component of the signal by using input capacitive coupling or by applying a voltage or current source to the inputs of the amplifier in a direction opposite to the offset voltage. Capacitive coupling has the disadvantage of requiring either external capacitors, which may be expensive, or, if combined on the substrate of an integrated circuit, will take up a large area on the die. In addition, the capacitors can have a limiting effect on the bandwidth of the input signals and also will not eliminate the effects of input offset due to mismatches in the amplifier components. The addition of voltage or current sources applied to the input signal to cancel the dc offset have the disadvantage that they only work for a fixed gain amplifier, since these signals are also amplified by the amplifier circuit.
It is therefore desirable to reduce the effects of input offset voltages on the output of an amplifier circuit while maintaining the capability of adjusting the gain.